Arcing is a luminous discharge of electricity across an insulating medium. An arc can cause a fire once the electrical energy from the arc becomes thermal energy. The heat generated by the thermal energy may then cause combustible material near the arc to ignite. Arcing occurs in wiring in two basic forms: parallel arcing and series arcing. Parallel arcing is an arc fault between two conductors, e.g., hot and neutral or hot and ground. An example of parallel arcing is when damage to the insulation of a power cable or cord between the hot and another (grounded or grounding) conductor allows an arc to jump across the damaged insulation between the hot and the other conductor. Series arcing is an arc established between the ends of a break (discontinuity) in a single conductor. Examples include loose terminations in a twist-on wire connector, loose device connections, and partially severed lamp cords, extension cords, etc. There is no data available about which type of arcing is more prevalent, nor where each type of arc is most likely to occur in a branch circuit. Therefore, a solution to preventing fires from arcs should address both series and parallel arcs that occur on branch wiring, plug-in power supply cords, etc.
There are multiple ways of detecting arc faults. Typically, the low frequency and the high frequency power spectrums of the load current are used in determining the presence of a series arc or parallel arc fault. Detection of zero crossings of the load current sine wave signal have been used to synchronize detection of the low and high frequency power spectrums in determining whether the measured load current is normal or from an arc fault. “Shoulders” are present at the zero current crossings during arcing conditions. Differential current measurement circuits, like those used in ground fault circuit interrupter (GFCI) protection circuits have been used to detect current imbalances that may be due to arc faults from a conductor to ground. AFCIs must be able to distinguish between normal arcing resulting from operation of a light switch or appliance motor, e.g., vacuum cleaner, but trip on detection of hazardous series arcs in a single conductor and/or parallel arc faults between conductors.
UL 1699 defines four relevant types of AFCIs as follows:                Arc-Fault Circuit Interrupter (AFCI)—A device intended to mitigate the effects of arcing faults by functioning to de-energize the circuit when an arc-fault is detected.        Branch/Feeder Arc-Fault Circuit Interrupter—A device intended to be installed at the origin of a branch/circuit or feeder, such as at a panel board. It is intended to provide protection of the branch/circuit wiring, feeder wiring, or both, against unwanted effects of arcing. This device also provides limited protection to branch circuit extension wiring. It may be a circuit-breaker type device or a device in its own enclosure mounted at or near a panel board.        Outlet Circuit Arc-Fault Circuit-Interrupter—A device intended to be installed at a branch circuit outlet, such as at an outlet box. It is intended to provide protection of cord sets and power-supply cords connected to it (when provided with receptacle outlets) against the unwanted effects of arcing. This device may provide feed-through protection of the cord sets and power-supply cords connected to downstream receptacles.        Combination Arc-Fault Circuit Interrupter—An AFCI which complies with the requirements for both Branch/Feeder and outlet circuit AFCIs. It is intended to protect downstream branch circuit wiring and cord sets and power-supply cords.        
The NATIONAL ELECTRICAL CODE® (NEC®) (a registered trademark of the National Fire Protection Association, Inc., Corporation Massachusetts, One Batterymarch Park, P.O. Box 9101, Quincy, Mass. 02269-9101) first mandated the use of AFCIs in 1999 for the protection of branch circuits supplying bedroom receptacle outlets. The effective date of implementation was Jan. 1, 2002. During the past several years, millions of AFCI circuit breakers have been installed in the load centers of new construction houses to protect bedroom circuits. However, these installations still represent only a small fraction of all residential circuits. The AFCIs installed to date have been of the Branch/Feeder type, and their value has been recognized by many organizations including UL, CSA, NASFM and CPSC.
In 2005, section 210.12(B) of the NEC was changed to mandate the use of combination AFCIs for the protection of branch circuits supplying bedroom outlets, with the continued use of Branch/Feeder AFCIs being permitted only until Jan. 1, 2008. Both of these AFCI types mitigate the effects of high current arcs at any point in the branch circuit wiring and in the branch circuit extension wiring. Both of these AFCI types protect against low current arcs in the branch circuit wiring. The Combination AFCI also has the task of low-current arc-fault protection for cord sets and power-supply cords connected to outlets for example, but not limited to, receptacles and light fixtures.
In 2008, section 210.12(B) of the NEC was changed to expand the use of Combination AFCIs as follows: “(B) Dwelling Units. All 120-volt, single-phase, 15- and 20-ampere branch circuits supplying outlets installed in dwelling unit family rooms, dining rooms, living rooms, parlors, libraries, dens, bedrooms, sun rooms, recreation rooms, closets, hallways, or similar rooms or areas shall be protected by a listed arc-fault circuit interrupter, combination-type, installed to provide protection of the branch circuit.”